1. Field of the Invention
The present invention relates to CCD (charge coupled device) linear sensor, CCD imaging devices such as a CCD linear sensor and CCD elements such as other devices utilizing CCDs or the like.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a structure of a final stage portion of a horizontal transfer register of a conventional CCD imaging device and an output section thereof.
Referring to FIG. 1, a horizontal transfer register 1 has a plurality of arrays of transfer sections 4 each composed of transfer electrodes, i.e., a storage electrode 3S and a transfer electrode 3T formed on a major surface of a semiconductor substrate 2 via an insulating layer, and transfers signal charges in the horizontal direction by means of two-phase drive pulses .phi.H.sub.1 and .phi.H.sub.2 (see FIGS. 2A and 2B). The transfer section 4 at the final stage of the horizontal transfer register 1 is connected to a floating diffusion region 7 via a horizontal output gate section 5 to which there is applied a fixed gate voltage V.sub.HOG. The signal charge from the horizontal transfer register 1 is transferred to the floating diffusion region 7, in which it is converted in the form of charge to voltage and then output from an output amplifier 8. In an output section 9, a reset gate section 11 to which a reset pulse .phi.RG (see FIG, 2C) is applied is formed between the floating diffusion region 7 and a reset drain region 10 in order to drain out the signal charge transferred to the floating diffusion region 7 to the reset drain region 10 to which there is applied a predetermined voltage V.sub.RD (see FIG. 2D).
In the output section 9 of FIG. 1, as shown by a waveform of a potential on the floating diffusion region 7 in FIG. 2D, when the reset gate section 11 is turned on (when the reset pulse .phi.RG is high in level), a potential V.sub.FD of the floating diffusion region 7 becomes equal to a potential V.sub.RD of the reset drain region 10 (V.sub.FD =V.sub.RD). However, a parasitic capacitance C.sub.1 exists between the floating diffusion region 7 and the reset gate section 11 so that, after the reset gate section 11 is turned off, the potential V.sub.FD of the floating diffusion region 7 does not equal to the potential V.sub.RD of the reset drain region 10 due to a capacitive coupling of the parasitic capacitance C.sub.1 and becomes lower than the potential V.sub.RD (V.sub.FD &lt;V.sub.RD). Considering this phenomenon from a waveform of a CCD output shown in FIG. 2E, after the reset gate section 11 is turned off, the CCD output becomes a potential (i.e., field-through potential) b which is lower than a reference potential (i.e., reset potential) a. A difference between the reference potential a and the field-through potential b is rendered a so-called coupling amount A.sub.1 which is caused by the parasitic capacitance C.sub.1.
It is desirable that the parasitic capacitance C.sub.1 is reduced in proportional to a capacity associated with the floating diffusion region 7 as the CCD imaging device is increased in sensitivity. More specifically, if the parasitic capacitance C.sub.1 is not reduced, then the coupling amount A.sub.1 is relatively increased, resulting in a dynamic range of a signal component being reduced. As shown in a diagram of potentials in FIG. 3, the potential in the floating diffusion region 7 after the reset gate section 11 is turned off is lowered (shown by a solid line) from the potential V.sub.RD (shown by a dashed line) of the reset drain region 10. As a result, signal charges are difficult to be transferred to the floating diffusion region 7 and the amount of signal charges to be transferred is reduced. A potential difference B.sub.1 in FIG. 3 corresponds to the coupling amount A.sub.1 in the waveform of the CCD output.
Furthermore, in order to obtain a higher resolution, a CCD linear sensor is arranged such that signal charges of odd-numbered photo sensing elements (pixels) and even-numbered photo sensing elements (pixels) are transferred to two charge transfer registers in a divided fashion and then output from the final stage in the mixed form.
As shown in FIG. 4, first and second horizontal transfer registers 24 and 25 each being formed of a CCD are disposed on respective sides of a photo sensing region 21 having an array composed of a plurality of photo sensing elements (pixels) S (S.sub.1, S.sub.2, S.sub.3, . . . ) through read-out gate sections 22 and 23. Signal charges of the odd-numbered photo sensing elements S.sub.1, S.sub.3, S.sub.5, . . . are read out to the first horizontal transfer register 24 and signal charges of the even-numbered photo sensing elements S.sub.2, S.sub.4, S.sub.6, . . . are read out to the second horizontal transfer register 25. Then, signal charges are transferred in one direction by two-phase drive pulses .phi.H.sub.1, .phi.H.sub.2 and .phi.H.sub.1 ', .phi.H.sub.2 ' shown in FIGS. 5A and 5B, respectively. Transfer sections HR.sub.1, HR.sub.2 of the final stages are applied with independent drive pulses .phi.H.sub.1 ', .phi.H.sub.2 ' which are synchronized with the drive pulses .phi. H.sub.1, .phi.H.sub.2 supplied to preceding transfer sections HR.sub.1, HR.sub.2. The transfer sections HR.sub.1, HR.sub.2 provided at the final stages of the first and second horizontal transfer registers 24, 25 are connected to a common floating diffusion region 27 via a common horizontal output gate section 26 to which there is applied a gate voltage V.sub.HOG. Then, signal charges read out from the even-numbered and even-numbered photo sensing elements and transferred by the first and second horizontal transfer registers 24, 25 are added together by the floating diffusion region 27, that is, the odd-numbered and even-numbered signal charges are alternately transferred to the common floating diffusion region 27, in which they are converted in the form of charge to voltage and then signals corresponding to odd-numbered and even-numbered pixels are sequentially and alternately output from an output amplifier 28 as shown by a waveform of a CCD output in FIG. 5D. In this output section, the signal charges transferred to the floating diffusion region 27 are discharged to a reset drain region 30 through a reset gate section 29 by a reset pulse .phi.RG which is supplied thereto and whose frequency is twice as high as those of the two-phase drive pulses .phi.H.sub.1, .phi.H.sub.2 and .phi.H.sub.1 ', .phi.H.sub.2 ' as shown in FIG. 5C.
In the CCD linear sensor, large capacitances of the first and second transfer sections HR.sub.1, HR.sub.2 in the horizontal transfer registers 24, 25 are driven by the drive pulses .phi.H.sub.1, .phi.H.sub.2 so that noise components caused by such capacitances are superimposed upon the output signal. Consequently, the waveform of the CCD output during the signal period, for example, is fluctuated. Further, when signal charges are read out by the two horizontal transfer registers 24 and 25, a difference known as "DC level difference" occurs between the signal level of the odd-numbered pixel (photo sensing element) and that of the even-numbered pixel (photo sensing element).
In the output section, the floating diffusion region 27 is formed within a well region and the well region is interconnected to the ground. In that event, the capacitance of the transfer sections HR.sub.1, HR.sub.2 is large so that, when the horizontal transfer registers 24, 25 are driven by the two-phase drive pulses .phi.H.sub.1, .phi.H.sub.2, a well region potential (so-called reference potential) Vo is momentarily fluctuated at negative-going edges t.sub.1, t.sub.2, t.sub.3, . . . of the drive pulse .phi.H.sub.1 or .phi.H.sub.2 which transfers the signal charges to the floating diffusion region 27 as shown in FIG. 5, to thereby produce a fluctuated component 15. Consequently, a noise component 16 is superimposed upon the output signal on the basis of this fluctuated component 15. Thus, the waveform during the signal period is not flat. Further, since the capacitance of the first transfer section HR.sub.1 is different from that of the second transfer section HR.sub.2, the noise component 16 is changed, thereby resulting in a difference (so-called DC level difference) being produced in the signal level of the odd-numbered pixel and that of the even-numbered pixel.
This signal level difference becomes conspicuous because the resolution of the CCD linear sensor is increased more and more. Therefore, each of the pixels must be corrected, which is very cumbersome in the prior art.